Titanium-based thermal ground plane

ABSTRACT

Titanium-based thermal ground planes are described. A thermal ground plane in accordance with the present invention comprises a titanium substrate comprising a plurality of pillars, wherein the plurality of Ti pillars can be optionally oxidized to form nanostructured titania coated pillars, and a vapor cavity, in communication with the plurality of titanium pillars, for transporting thermal energy from one region of the thermal ground plane to another region of the thermal ground plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C Section 120 ofcopending and commonly assigned U.S. Utility application Ser. No.14/338,033, filed on Jul. 22, 2014, by Noel C. MacDonald, Carl D.Meinhart, Changsong Ding, Payam Bozorgi, Gaurav Soni, and Brian D.Piorek, entitled “TITANIUM-BASED THERMAL GROUND PLANE,” attorneys'docket no. 30794.0284USC1 (2009-015-3), which application is acontinuation under 35 U.S.C Section 120 of U.S. Utility application Ser.No. 13/055,111, filed on Jan. 20, 2011, by Noel C. MacDonald, Carl D.Meinhart, Changsong Ding, Payam Bozorgi, Gaurav Soni, and Brian D.Piorek, entitled “TITANIUM-BASED THERMAL GROUND PLANE,” attorneys'docket no. 30794.284-US-WO (2009-015-2), which application claims thebenefit under 35 U.S.C. § 365(c) of PCT International Application SerialNo. PCT/US2009/051285, filed on Jul. 21, 2009, by Noel C. MacDonald,Carl D. Meinhart, Changsong Ding, Payam Bozorgi, Gaurav Soni, and BrianD. Piorek, entitled “TITANIUM-BASED THERMAL GROUND PLANE,” attorneys'docket no. 30794.284-WO-U1 (2009-015-2), which application claims thebenefit under 35 U.S.C. 119(e) of U.S. Patent Application Ser. No.61/082,437, filed on Jul. 21, 2008, by Noel C. MacDonald, Carl D.Meinhart, Changsong Ding, Payam Bozorgi, Gaurav Soni, and Brian D.Pionek, entitled “TITANIUM-BASED THERMAL GROUND PLANE,” attorneys'docket no. 30794.284-US-P1 (2009-015-1);

all of which applications are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under Grant No.W9113M-04-01-0001 awarded by the U.S. Army. The Government has certainrights in this invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to semiconductor devices, and, more particularly,to thermal ground planes used with semiconductor devices.

2. Description of the Related Art

Electronics employing various semiconductor devices and integratedcircuits are commonplace and are subjected to various environmentalstresses. Applications of such electronics are extremely widespread, andutilize various different types of semiconductor materials.

Operating environments for electronic devices can be extremely harsh.Large temperature changes, gravitational forces, and shock resistanceare required for electronic devices to perform their functions properly.Further, as semiconductor processing and materials have advanced,semiconductor capabilities and heat dissipation have also increased.

Typically, semiconductor devices and integrated circuits are thermallybonded to heat sinks to dissipate heat generated by the semiconductordevices during operation. There are various problems with suchapproaches, such as ensuring such assemblies can survive theenvironmental and structural requirements of the operating environmentand ensuring that the overall weight and size of the heat sink/deviceassembly fits within the design envelope of the application. Further,materials used for the heat sink must not adversely affect the device,even though the materials are dissimilar in terms of thermalcoefficients of expansion. Such differences usually lead to more complexheat sink designs that are more difficult to incorporate into theapplication for the semiconductor devices.

It can be seen, then, that there is a need in the art for coolingsemiconductor devices.

SUMMARY OF THE INVENTION

The present invention describes a Thermal Ground Plane (TGP) for coolingsemiconductor devices, integrated circuits, high-power electronics,radar systems, laser radiation sources, and the like. The TGP isfabricated using Titanium (Ti) and optionally Titania (TiO₂) processingtechnology of the present invention, including Nano-Structured Titania(NST) and wafer-scale processes. Optionally, composite materials usinghigh thermally-conductive materials can be used to increase thermalconductivity. These materials include but are not limited to gold,copper, and the like.

The present invention discloses the micro/nano scale processes that canbe used to form NST, which is a super-hydrophilic wick material, onmicro-scale, deep etched titanium pillars. An array of Ti or Ti/NSTpillars forms a wicking structure for the TGP. The Ti or Ti/NST wickingstructure can be tailored to the application by changing the density,position, pitch, spacing (or gap), and height of the deep etchedtitanium pillars. In addition, the degree of oxidation can be used totailor the wicking structure. Optionally, composite structuresconsisting of highly conductive materials can be used to furtherincrease the thermal conductivity of the wicking structure.

The present invention comprises a titanium sheet (or foil) with anintegrated array of titanium micro-scale pillars of controlleddimensions, which can be coated with NST, and cavities to support thechips; the top sheet is bonded or laser welded to a second sheet oftitanium. The area of the TGP is greatly variable, and can be less than1 cm×1 cm or greater than 10 cm×20 cm. Very large TGPs can befabricated, for instance, using large screen LCD processing equipment,and large area etch processes, or other machining techniques.

A thermal ground plane in accordance with the present inventioncomprises a titanium substrate comprising a plurality of pillars ofcontrolled dimension, wherein the plurality of pillars can be oxidizedto form nanostructured titania, and a vapor cavity, in communicationwith the plurality of titanium pillars, for conducting thermal energyfrom the titanium substrate.

Such a thermal ground plane further optionally comprises the titaniumsubstrate that can be optionally be thinned in at least part of an areaof the substrate opposite the plurality of pillars, the vapor cavitybeing enclosed using a second substrate (which can optionally beconstructed from titanium), the plurality of titanium pillars beingformed using optionally titanium inductively-coupled-plasma etching, atleast one characteristic of the plurality of pillars can be controlledand optionally varied within the plurality of pillars to adjust athermal transport of the thermal ground plane, and the at least onecharacteristic being selected from a group comprising a height, adiameter, a spacing (or gap), an amount of oxidation, a pitch of theplurality of pillars, a composition of the interior pillar regions whichmay include materials other than titanium, and a composition ofmaterial(s) which may include but is not limited to Ti, TiO₂, Au, or Cuapplied to the pillar surface to control surface physical propertiesincluding wettability.

Such a thermal ground plane further optionally comprises the pillarsconsisting of a composite structure of Ti/Au or Ti/Au/Ti or othersuitable materials that can increase the thermal conductivity of thewicking structure.

A method of forming a thermal ground plane in accordance with thepresent invention comprises etching a titanium substrate to form aplurality of titanium pillars, optionally oxidizing the titaniumsubstrate to form nanostructured titania on the plurality of titaniumpillars, and forming a vapor cavity in communication with the pluralityof titanium pillars.

Such a method further optionally comprises the titanium substrate thatcan optionally be thinned in at least part of an area of the substrateopposite the plurality of pillars, the vapor cavity being enclosed usinga second substrate (which can optionally be constructed from titanium),the plurality of titanium pillars being formed using optionallytitanium-inductively-coupled-plasma etching, at least one characteristicof the plurality of pillars can be controlled and optionally variedwithin the plurality of pillars to adjust a thermal transport of thethermal ground plane, and the at least one characteristic being selectedfrom a group comprising a height, a diameter, a spacing (or a gap), anamount of oxidation, a pitch of the plurality of pillars, and acomposition of material(s) which may include but is not limited to Ti,TiO₂, Au, or Cu applied to the pillar surface to control surfacephysical properties including wettability.

Such a method further optionally comprises the pillars consisting of acomposite structure of Ti/Au or Ti/Au/Ti or other suitable materialsthat can increase the thermal conductivity of the wicking structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates a schematic of a preferred embodiment of the Ti-basedthermal ground plane of the present invention;

FIG. 2 illustrates a schematic of an embodiment of the Ti-based thermalground plane in accordance with one or more embodiments of the presentinvention;

FIG. 3A illustrates a picture of a laser welded Ti thermal ground planeand FIG. 3B illustrates a weld/joint, in accordance with one or moreembodiments of the present invention;

FIG. 4 is a scanning electron microscope image of the titanium pillarmicrostructure in accordance with one or more embodiments of the presentinvention: (A) shows array wicking structure, (B) shows some dimensionsthat can be controlled including height, diameter, and spacing (denotedas gap, g), (C) is a closeup showing the nano-structured-titania (NST);

FIG. 5 illustrates side-views of typical processing steps used inaccordance with one or more embodiments of the present invention;

FIG. 6A and FIG. 6B illustrate alternative composite pillar structuresin accordance with one or more embodiments of the present invention,wherein FIG. 6A illustrates a pillar and a material and FIG. 6Billustrates a pillar, a material, and an outer layer;

FIG. 7 illustrates a plot of characteristic wetting speeds in a typicalwick embodiment of the present invention;

FIG. 8 illustrates a plot of effective thermal conductivity of the TGPas a function of temperature at the heat source in accordance with oneor more embodiments of the present invention; and

FIG. 9 illustrates a flow chart of the formation of one or moreembodiments of the Ti-based TGP in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

Titanium is a material that is used in many applications that aresubjected to harsh environments, including stealth systems and aerospacevehicles. Commercial applications for titanium include oil well drillingequipment, transportation, shipping, and chemical manufacturing.Titanium and titanium alloys can provide excellent bio-compatibility andhave achieved broad acceptance for use in medical and biologicalapplications, including hip replacements, dental implants, and packagingof implant devices, sensors and pacemakers. Micro-machined titaniumsubstrates with integrated Nano-Structured Titania (NST) can also beused to make more robust, shock resistant Thermal Ground Planes (TGPs)including laser welded Ti-TGP packaging.

Micro-machined (i.e. microfabricated) Ti pillars can be manufacturedwith controlled dimensions (height, diameter, and spacing) to engineerthe wicking structure for optimum performance and customized to specificapplications. Titanium can be oxidized to form nanostructured titania(NST). Titanium is a material that provides for deposition inconjunction with high-thermally conductive materials. For example,materials such as gold or copper can be electroplated on titanium, andfurthermore titanium can be deposited onto gold or copper.

Titanium is one of the few materials that can be microfabricated usingcleanroom processing techniques, macro-machined in a machine shop, andlaser welded to form a hermetic seal. The combination of thesemanufacturing techniques creates a unique method for fabricating TGPs.

The present invention describes fabrication of titanium-based ThermalGround Planes (TGPs). The present invention includes two substrates (ofwhich one or more can be constructed using titanium), one substratesupports an integrated super-hydrophilic wicking structure and a secondsubstrate consists of a deep-etched (or macro-machined) vapor cavitythat can optionally be laser welded to the wicking structure to form theTGP.

Schematic View

FIG. 1 illustrates a schematic of a preferred embodiment of the Ti-basedthermal ground plane of the present invention.

The Thermal Ground Plane (TGP) 100 of the present invention typicallycomprises a substrate 102, which further comprises pillars 104 that forma wicking structure 105. Typically, the substrate 102 is titanium, andtypically the characteristic dimension of the titanium substrate 102 is25-500 um thick, and can span 1 cm more than 20 cm in the lateraldimension. In other embodiments, substrate 102 could be formed fromother materials, such as but not limited to Al, Cu and the like, eitheralone or as a composite with titanium.

Typically, the pillars 104 are titanium, but can also be other materialsin accordance with the present invention, including nano-structuredtitania (NST), a composite of titanium with other metals such as gold orcopper, or other materials either alone or as a composite with titanium.In the present description, the discussion is with respect to titaniumpillars 104, however, it is understood that other materials can be usedas well in accordance with the present invention.

The titanium-based pillars 104 are typically nominally 5-200 microns inheight, and nominally 5-500 microns in diameter. The spacing between thepillars 104 (i.e. the gap) can be nominally 1-500 microns. Thesedimensions of the pillars, e.g., height, diameter, and spacing (or gap),are controlled and optionally varied within the plurality of pillarswithin the TGP 100 in order to maximize TGP performance. For instance,the dimensions can be designed such that viscous losses are minimizedand capillary forces are maximized in order to improve TGP performance.Although the dimensions, or characteristics, of the pillars 104 can varythroughout the TGP 100, the characteristics can vary locally within theTGP 100 or can vary from one pillar 104 to another pillar 104, asdesired for a given application or use of TGP 100, without departingfrom the scope of the present invention.

A second substrate 106, and structural members 110 (which can be part ofeither substrate 102, second substrate 106, or separate members 110),are combined to form a vapor cavity 108. The first substrate 102 istypically processed in accordance with the present invention to createthe wicking structure 105. The first substrate 102 and second substrate106 are, again, typically titanium, however, either substrate 102 or thesecond substrate 106 can be of a different material, or differentmaterials than each other, if desired.

Typically, the structure 100 is ˜1 mm thick, but can be thicker orthinner depending on the desires of the designer or the designrequirements of overall structure 100. The thickness of first substrate102 is typically 25-500 microns.

The super-hydrophilic 3D wicking structure 105 can also comprisetitanium pillars 104 and can be optionally coated with TiO₂(Nano-Structured Titania). The pillars 104 in the array 105 can be 5 μmin diameter and 40 μm in height, however, the pillars can be ofdifferent diameters and heights depending on the design of structure100, and the shapes of the pillars can be changed, based on the heattransport characteristics desired or required by structure 100.

The pillars can optionally be constructed from a composite of thermallyconductive materials. These materials include but are not limited to Ti,TiO₂, Au, or Cu applied to the pillar surface to control surfacephysical properties including wettability. Typically, vapor cavity 108is approximately 100-500 μm thick, however, again, this thickness canvary depending on the desires for or requirements of structure 100. Forexample, wicking structure 105 thermal conductivities of >100 W/mK andwicking inertial force (g-force) of greater than 20 G's are possiblewith the present invention. The vapor cavity is sealed by secondsubstrate 106, where second substrate 106 hermetically seals the volumedescribed by vapor cavity 108. Hermetic sealing of less than 0.1% fluidloss per year at 100° C. is possible using laser welding 114 of thewicking structure 105 to the second substrate 106, forming vapor cavity108.

As heat 116 is generated by a heat source, first substrate 102 andwicking structure 105 transfer the heat to the fluid 118, typicallywater, contained in wicking structure 105 in liquid phase. Heat istransferred to the fluid 118, which transforms the fluid 118 from liquidphase into vapor phase through latent heat of evaporation, transferringthe heat from heat source 116, which can be a semiconductor device,high-power electronics, radar systems, laser radiation sources, and thelike, or other heat source. The evaporation of fluid 118 from wickingstructure 105 creates a region void of liquid in wicking structure 105.This void of liquid creates a capillary force through surface tensionthat draws liquid through the wicking structure 105, and allows vapor tobe transported within the vapor cavity 108 as a result of a pressuregradient. The vapor is transported through the vapor cavity 108. Thevapor condenses and returns to a liquid state, thereby releasing thelatent heat of evaporation at the location of condensation near heatsink 120. The liquid is then transported through the wicking structure105 from the cooler region near heat sink 120 towards the hot regionnear the heat source 116, thereby completing the thermal transportcycle.

Similarly, structure 100 can be designed to transfer heat out ofstructure 100, e.g., act as a cooling source at one area of structure100. For example, and not by way of limitation, the heat sink 120 canact as a removal area of heat for a device attached in that area, andthe heat source 116 can remove of the heat transferred through vaporcavity 108. In essence, structure 100 can transport thermal energy ineither direction, or act as a constant temperature source, for devicesattached to structure 100, as desired.

The thickness of substrate 102 can be varied to be thinner at thelocation of heat source 116 and thinner at the location of heat sink120, and thicker in other regions, which can be used for increased heattransfer, as a mounting location or indicia for the heat source 116, orother reasons, such as increasing structural integrity, as desired forthe application of structure 100. The varied thickness of substrate 102can also facilitate thermal matching, by reducing thermally-inducedstresses imparted by substrate 102 to devices mounted to the TGP. So,for example, thermal matching of 10% for GaAs, Si and GN using the 25 μmthick substrates to support the semiconductor devices is possible withinthe scope of the present invention. This relatively small thickness ofsubstrate 102 can be supported by thicker beams or pillars that extendfrom first substrate 102 to second substrate 106 through the vaporcavity 108, if such support is necessary for a given heat source 116.Further, a larger portion or the entirety of substrate 102 can bethinned to any desired thickness to increase thermal transfer if desiredor needed for a given application of structure 100.

The titanium pillars 104 can be formed using a variety of methods. Inone embodiment, the pillars 104 are etched using inductively-coupledplasma etching from a titanium substrate. In other embodiments, thepillars 104 can be macro-machined from the titanium substrate 102. Inother embodiments, the pillars 104 can be grown on top of the titaniumsubstrate 102. In other embodiments, the pillars can be stamped into thetitanium substrate. In other embodiments, the pillars can be formed byselective laser ablation.

The TGP 100 is formed by attaching the titanium substrate to astructural backplane, which can be manufactured from a variety ofmaterials. In a preferred embodiment, the structural backplane can bemachined from a second titanium substrate 106. The machining processcould either be micro-machining or macro-machining. In a preferredembodiment, the two titanium substrates 102 and 106 are laser weldedtogether at welds 114 to form a hermetically-sealed vapor cavity 108.

Again, the vapor cavity 108 typically spans the lateral dimension of theworking portion of the TGP 100, but can take various forms as desired.In a preferred embodiment, the vapor cavity 108 can have a depth of 10microns to several millimeters, with a nominal thickness of 100microns-1 millimeter. Judicious design of the wicking structure 105allows for high mass flow rates of fluid 118 to be transported andthereby large amounts of heat to be transported. For example, largeheight and large spacing of the pillars 104 will reduce viscous losses.In addition, smaller spacing of the pillars 104 will increase capillaryforces. Judicious choices of these parameters throughout the TGP 100will provide optimum TGP 100 performance for a given application of TGP100.

In some embodiments, the titanium pillars 104 can be oxidized to formnano-structured titania (NST). NST can be used to increase wettabilityand thereby increase capillary forces, and enhance heat transfer, withinTGP 100.

As shown in FIG. 1, structure 100 comprises substrate 102 having athickness of 250 microns, pillars 104 having a height of 40 microns, avapor cavity 108 having a height of 100 microns above the pillars 104,and a second substrate 106 having a thickness of 200 microns. Further,pillars 104 are varied in spacing throughout wicking structure 105.These are typical heights and thicknesses for structure 100, andstructure 100 can comprise other heights and thicknesses withoutdeparting from the scope of the present invention.

FIG. 2 illustrates a schematic of an embodiment of the Ti-based thermalground plane in accordance with one or more embodiments of the presentinvention.

In structure 200, backplane 202, which is typically macro-machined butcan be formed using other methods described herein, comprises one ormore mechanical standoffs 204, and is attached to substrate 102 toenclose wicking structure 105 and vapor cavity 108. Typically, backplane202 is laser welded 114 to a micro-fabricated wicking structure 105 onsubstrate 102, and mechanical standoffs 204 can be bonded to thesubstrate 102, but other methods of attaching backplane 202 andstandoffs 204 can be used if desired. Mechanical standoffs 204 can bedesigned to increase the structural integrity of structure 200, whichcan be important for TGPs 200 with large lateral dimensions. Further,the use of mechanical standoffs 204 provides for additional engineeringof substrate 102, including thinning substrate 102, since mechanicalstandoffs 204 allow for additional support of substrate 102 throughoutthe structure 200.

FIG. 3A illustrates a picture of a laser welded Ti thermal ground planein accordance with one or more embodiments of the present invention.

Structure 300 is shown, where wicking substrate 102 and joint 302 isshown between wicking substrate 102 and a second substrate 106 is shown.The second substrate 106 can optionally consist of the trench or cavity.Joint 302 is typically a laser weld 114, as shown in detail, tohermetically seal substrate 102 to second substrate 106 and form vaporcavity 108.

For continuous operation, the working fluid and the wicking structuremust be in communication with a vapor cavity and sealed such that theinternal mechanism of the thermal ground plane 300 is isolated from theexternal environment to avoid vapor loss and system contamination. Theperformance of the TGP 300 therefore significantly depends on thequality of packaging. A major problem with conventional packagingtechniques for such structures, such as high-temperaturethermo-compression and flip chip bonding, is the degradation ofreliability caused by the excess stress due to thermal mismatching. Toeliminate the stresses which occur at high temperature, laser welding114 is used to rapidly apply heat at a small region of the joint 302instead of heating the entire device to hermetically weld the titanium.

In one embodiment, a pulsed wave YAG laser (Neodymium-Doped YttriumAluminum Garnet, Nd: Y3Al5O12) with a wavelength of 1064 nm can be usedto weld the backplane 106 to the substrate 102. Such a laser can befocused to a very small area, for example 400 microns in diameter, tolocally heat the material to the melting point. Given sufficient laserpower and linear translation speed, for instance 1.5 J at 2 mm/sec, thesubstrate 102/backplane 106 joint 302 is welded yet the total energyabsorbed is quickly dissipated by the bulk material such that nearbyregions of the substrate 102/backplane 106 remain physically unaffectedby the heat injected into the device by the welding process.

In other embodiments, other local sources of heat can be used to performprecise welding of the substrate 102 to the backplane 106. For instance,a CO₂ or other types of lasers can be used. In another embodiment, asmall gap between closely placed electrodes can be used to create aprecisely positioned electrical arc which can provide the required heat.In another embodiment, a closely placed electrode can be used to directan electrical arc between itself and the substrate 102/backplane 106joint 302, thereby adding the required amount of heat to achieve localwelding. In yet another embodiment, substrate 102 and backplane 106 canbe thermally bonded by depositing Au on each member and thermallybonding the Au on each member at joint 302.

SEM Images

FIG. 4 is a scanning electron microscope image of the titanium pillarmicrostructure in accordance with one or more embodiments of the presentinvention.

FIG. 4A shows pillars 104 in an array structure to create an embodimentof wicking structure 105. As shown in FIG. 4B, the diameter “d” 304,spacing or gap “g” 306, and height “h” 308 are fairly uniform, however,diameter 304, gap 306, and height 308 of the pillars 104, individually,locally, or collectively can be controlled and/or optionally variedwithin the structure 100 plurality of pillars to optimize theperformance of the TGP 100.

The pillars 104 are arranged in an array such that the diameter 304,spacing 306, and/or height 308 between the pillars 104 are controlledand optionally varied to allow sufficient liquid 118 flow velocitybetween the pillars 104. The flow velocity of liquid 118 is controlledby reducing viscous losses while simultaneously providing optimalsurface area in order to draw the fluid 118 at a proper speed from thecool region 120 to the hot region 116 of the resulting TGP 100. Sincethe evaporation, adiabatic, and condensation regions of the TGP 100perform separate functions within the TGP: evaporation, vapor and liquidphases of fluid 118 transport, and condensation, respectively, thepillar geometry, composition, and distribution can be specificallydesigned to perform optimally in each of these regions. Further, thepillars 104 in the wicking structure 105 can be in an array format, orin any random, pseudo-random, or otherwise structured design withoutdeparting from the scope of the present invention.

FIG. 4C shows a SEM image 310 of nanostructured titania (NST) 312 etchedinto a pillar 104. The pillar 104 is oxidized to produce hair-like NST312 with a nominal roughness of 200 nm. Other embodiments may includeNST 312 with a nominal roughness of 1-1000 nm. The hair-like NST 312structure enhances the wetting properties of Ti pillars 104 whichincreases the working fluid 118 wetting performance within the wickingstructure 105, and the overall heat transport properties of the TGP 100.

Array Processing

FIG. 5 illustrates side-views of typical processing steps used inaccordance with the present invention.

Step 1 shows a bulk titanium wafer 400, which is polished sufficientlyto allow for the desired lithographic resolution. Step 2 illustrates amasking material 402 that is deposited and patterned. Typical maskingmaterials are TiO₂ or SiO₂. The masking material 402 is typically anoxide, which is deposited using CVD methods for SiO₂ and PVD methods forTiO₂. Step 3 illustrates a pattern defined on the surface of maskingmaterial 402 using a photoresist 404, and step 4 illustrates an etching406 to transfer the pattern into the masking material 402.

Step 5 illustrates deeply etching the substrate 400 using etch 408. Etch408 is typically a Titanium Inductively coupled plasma etching processknown as a TIDE process. The TIDE process is described in “TitaniumInductively Coupled Plasma Etching” by E. R. Parker, et al., J.Electrochem. Soc. 152 (2005) pp. C675-83, which is incorporated byreference herein.

Step 6 illustrates masking material 402 being removed from substrate400. Masking material 402 must be removed if formation of NST is desiredon the top surface 410 of substrate 400. Likewise, if NST features aredesired on sidewalls 412, etch mask products must be cleared from thesidewalls 412. However, if the formation of NST is not desired on topsurface 410, then mask 402 can be left in place if desired. Pillars 414are now defined within substrate 400. The removal of masking materialfrom top surface 410 and/or sidewalls 412 is typically performed usingdry oxide etch chemistries, e.g., CF₄/O₂ plasma etching.

Step 7 illustrates substrate 400 being oxidized, which is typicallyperformed in hydrogen peroxide between 80° C.-100° C. NST 416 is thenformed on the titanium pillars 414 of substrate 400.

The aspect-ratio (height to width) of pillars 414 and the pitch (angleof the pillars 414 with respect to the substrate 400) can be controlledby etching profiles and techniques, and the hydrophilic capabilities ofeach pillar 414 can be controlled by the amount and/or depth of NST 416formed on each pillar 414, e.g., whether the NST 416 is formed on thetop surface 410, how long the pillars 414 are oxidized, etc.

Composite Pillar Structure

FIG. 6A and FIG. 6B illustrate alternative composite pillar structuresin accordance with one or more embodiments of the present invention.

Pillar 104 is shown, which typically comprises titanium. Pillar 104 canbe surrounded by a highly thermally conductive material 600, such as butnot limited to Au or Cu. Optionally, an outer layer 604 can be added tocontrol wettability, such as but not limited to Ti or nanostructuredtitania. The use of material 600 and/or outer layer 604 providesflexibility in controlling thermal conductivity and wettingcharacteristics in the wicking structure 105 independently.

In the hot and cold regions of the TGP 100, heat transfer between theworking fluid and the TGP is affected by the thermal conductivity of thepillars 104 in these regions. Since overall TGP 100 performance isimproved by a) improving the wetting speed of the working fluid 118, andb) improving the heat transfer properties of the pillar array 105, theseparameters can be optimized independently to optimize TGP performance.

Since wetting speed can be improved by providing a super hydrophilicsurface on the outside area of the pillars 104, a rough surface such asthat arising from oxidized Ti NST 604 can be added to each pillar 104 asshown in FIG. 6B. However, since neither NST nor Ti provides optimalheat conduction through the volume which they comprise, a material 600layer of Au or other material of improved heat transfer properties canbe in communication with the Ti or NST outer layer 604 to improve theheat transport properties of each pillar 104.

Materials providing improved pillar 104 heat transfer properties can beadded by, for example, thermal evaporation processes such as thosedriven by either tungsten filament heaters or electron beam sources;molecular beam epitaxy, chemical vapor deposition processes, orelectroplating processes, or other methods.

In one embodiment, a concentric layer of Au or other heat-conductingmaterial 600 can be added between the outer Ti/NST layer 604 and aninner core of Ti pillar 104. In another embodiment, a Ti/NST layer 604can be added to a pillar 104 consisting entirely of Au or otherheat-conducting material 600. In another embodiment, a Ti/NST layer 604can be added to a microcomposite structure comprised at least partiallyof Au or Cu or other thermally-conducting material.

Wetting Speeds

FIG. 7 illustrates a plot of characteristic wetting speeds in a typicalwick embodiment of the present invention.

FIG. 7 illustrates results 700 of an NST pillar 104 array, show a 70%increase in wetting velocity when using NST coated pillars.Characteristic speeds are of order centimeters per second. Results 702show a titanium pillar 104 array wetting velocity.

Heat transport from the hot to cool region of the TGP 100 is provided byevaporation of liquid-phase fluid 118 into vapor-phase fluid 118, andthe transport of vapor-phase fluid 118 from hot region 116 to the coldregion 120 of the substrate 102. Simultaneously, the liquid-phase offluid 118 is transported through wicking structure 105 from cold region120 to hot region 116, which results from capillary forces, therebycompleting the fluid transport cycle for transport of heat through theTGP. The wetting speed of fluid 118 through wicking structure 105,coupled with the height 308 of the wicking structure 105, determines therate of mass transfer of fluid 118, and therefore the maximum rate atwhich heat can be transported through the TGP 100.

The heat transfer properties of the TGP 100 are therefore affected bythe wetting speed of liquid-phase fluid 118 through wicking structure105 (which is in communication with wicking substrate 102), wherebyhigher wetting speeds provide higher thermal transport of the TGP 100.

The wetting speed through wicking structure 105 was determined byobservation. Wetting speed follows the Washburn equation which describesthe associated wetting dynamics for the case of θ=0°. The wetting speeddecreases with increasing wetting distance x as expected, due toincreasing viscous resistance as the wetting path becomes larger. TheNST-coated pillars 104 improve substrate wetting speed over the entirerange of wetting distance x in comparison to pillars 104 which are notcoated with NST. This indicates the application of NST to the pillarswill improve the heat transport properties of the TGP.

Effective Thermal Conductivity

FIG. 8 illustrates a plot of the effective thermal conductivity of theTGP as a function of temperature at the heat source in accordance withone or more embodiments of the present invention.

Graph 800 shows heat carrying capacity of a TGP 100 embodiment comprisedof Ti pillars 104 coated with NST, and was evaluated by holding the hotregion (i.e. hot side) 116 of the TGP 100 at several temperatures whilemaintaining the temperature of the cold region (i.e. cold side) 120 ofthe TGP 100 constant. Effective thermal conductivity k of the TGP 100increases as temperature is increased on the hot region (side) 116 ofthe TGP 100. In the depicted TGP 100 configuration, heat pipe ‘dryout’occurs at temperatures greater than 105 C. Dryout occurs due to the lackof capillary-driven flow of liquid-phase fluid 118 through wickingstructure 105 sufficient to replenish the evaporated fluid 118 at hotregion 116. By varying the design parameters of the TGP 100, includingpillar 104 diameter 304, height 308, and spacing 306, the dryouttemperature and overall heat carrying capacity of the TGP 100 can beoptimized for various applications. In one embodiment, at least oneparameter of the TGP 100 design can be controlled and optionally variedwithin the plurality of pillars 104 to increase or decrease the dryouttemperature to match a particular application. In another embodiment, atleast one parameter of the TGP 104 design can be controlled andoptionally varied within the plurality of pillars 104 to increase ordecrease the overall heat carrying capacity of the TGP to match aparticular application.

This demonstrates that the TGP 100 of the present invention can achieveat least 350 W/m-K of effective thermal conductivity.

Modeling of the Invention

The performance of the NST wicking structure and the packaged TGP hasbeen modeled using computer software. The TGP of the present inventionuses micro-scale NST coated Ti pillars as the wicking structure. Thedistribution and density of the pillars 104 that form the wickingstructure 105 are variables that determine the performance of the TGPstructure 100. By using simulation and modeling for a number ofpillararray designs, the present invention can produce designs thatdeliver optimal effective thermal conductivity across the entire wickingstructure, or optimized for specific locations of one or a plurality ofhot regions 116 and one or a plurality of cold regions 120 on substrate102.

Numerical simulations of the capillary-driven fluid motion, vaporphasetransport, heat transfer, and stress analysis was performed using COMSOLMultiphysics (COMSOL, Stockholm, Sweden) finite element software. Thecapillary-driven fluid motion through the NST wicking structure 105 wasmodeled using surface tension, the Navier-Stokes equation, andcontinuity. The level-set method was used to model the liquid/vaporinterface. The rate of liquid evaporation multiplied by the heat ofvaporization was balanced with the heat adsorbed inside the TGP. Thisrate was used as a sink term for the conservation of mass equation ofthe liquid phase, and as a source term for the conservation of massequation of the vapor phase.

The vapor-phase transport was modeled using the Navier-Stokes equation.At the cool end of the TGP, the rate of condensation divided by the heatof evaporation was balanced by the rate of condensation and thesource/sink terms for the mass conservation equations of theliquid/vapors phases, respectively. The temperature distribution wasmodeled using the energy conservation equation. The simulation model wascorrelated with experiments to understand transport mechanisms. Theresults can be used to optimize the performance of the TGP for a varietyof geometries and operating conditions.

Simulations on thermal mismatch were also performed to determine thesuitability of a given TGP design for a specific application. Thethermal expansion coefficients (TEC) for semiconductor materials varysignificantly, for example (in units of 1 e⁻⁶/° C.), Silicon is 2.6,GaAs is 6.9, and GaN is 3.2. Unfortunately, the TECs do not match wellwith potential conducting materials for TGPs, e.g., titanium (Ti) is8.5, copper (Cu) is 13.5, and aluminum (Al) is 23. In order to minimizethermally-induced stresses between the TGP and the semiconductor-baseddevice, it is desirable to match the TEC between the TGP and the chipwithin 1%.

The TECs are material specific. In principle, it may be possible todesign a TGP that thermally matches one semiconductor, such as Si,however, it would be very difficult to design a universal TGP that canmatch several different semiconductor materials simultaneously. Insteadof matching TECs directly, the present invention uses an alternativeapproach that universally reduces thermally induced stresses for allsemiconductor materials, simultaneously.

The induced stress from two dissimilar materials bonded together can beapproximated to first order (Eq. 1) by: (a) assuming the materials donot bend appreciably, (b) matching the total strain (i.e. the strain dueto thermal expansion and the strain due to normal stress), and (c)equating equal and opposite forces due to normal stresses, such asσ₁t₁=σ₂t₂, where σ₁ & σ₂ are the normal stress from material 1 & 2,respectively. The thickness of materials 1 & 2 are denoted by t₁ & t₂,respectively.

$\begin{matrix}{\sigma_{1} = {\left( {\alpha_{2} - \alpha_{1}} \right)\Delta \; {T\left\lbrack {\frac{1}{E_{1}} + {\frac{1}{E_{2}}\frac{t_{1}}{t_{2}}}} \right\rbrack}^{- 1}}} & (1)\end{matrix}$

As an example, if material 1 is the semiconductor chip, and material 2is the TGP, for a given temperature change, ΔT, the normal stress in thesemiconductor can be reduced by a number of factors. Clearly, one way toreduce σ₁ is to reduce the thermal mismatch, (α₂−α₁).

However, the present invention utilizes another method, which is toreduce thermally-induced stresses by maximizing the ratio t₁/t₂. SinceTi is a ductile material, with moderate strength, and is durable andcorrosion resistant, it can be micromachined with ˜25 μm thick layersthat interface with the semiconductor chip, while maintaining structuralintegrity. This not only provides efficient heat conduction from thechip to working fluid inside the TGP, but dramatically decreases thermalstresses, for all types of semiconductor materials simultaneously.

Considering a challenging scenario, where the temperature difference isΔT=50° C., and assuming a t₁=500 μm thick Si wafer (chip 110) is bondedto a t₂=1 mm thick solid Cu TGP. From Eq. (1), the induced thermalstress in the Si chip would be σ₁=54 MPa. By comparison, the same t₁=500μm thick Si wafer bonded to a t₂=25 μm thick Ti sheet (first substrate102) would induce a thermal stress in the Si chip of σ₁=1.5 MPa, whichis a 36 fold reduction in stress, compared to the solid 1 mm thick Cusubstrate.

For reference, σ₁=1.5 MPa is the same level of thermally-induced stressthat a 1 mm thick Cu substrate would impart on the same Si chip, if itseffective TEC was only α_(eff)=2.8, which is similar (within 8%) to theactual TEC for Si, α_(Si)=2.6. Further, the titanium sheet would weighless than the comparable copper thermal ground plane, which makes theapproach of the present invention more desirable in applications whereweight is a factor, e.g., space flight.

Similar results are derived for other semiconductor materials. Theeffective TEC of the t₂=25 μm Ti TGP for GaN (α_(GaN)=3.4) isα_(eff)=3.4 (i.e. matches GaN to within 6.3%). Similarly, the effectiveTEC of the Ti TGP for GaAs (α_(GaAs)=6.9) is α_(eff)=7.03 (i.e. matchesGaAs to within 1.8%).

In order to access the applicability of Eq. (1) to describe thethermally-induced stresses that our proposed Ti-based TGP would imparton a Si chip, a 2-D numerical simulation was conducted (COMSOLMultiphsyics V3.3; COMSOL, Inc., Stockholm, Se) to estimate a typicaltemperature distribution in the TGP.

Process Chart

FIG. 9 illustrates a flow chart of the formation of one or moreembodiments of the Ti-based TGP in accordance with the presentinvention.

Box 900 illustrates forming a plurality of titanium pillars on atitanium substrate.

Box 902 illustrates thermally coupling a vapor cavity with the pluralityof titanium pillars.

Box 904 illustrates containing a fluid within the vapor cavity and thetitanium pillars.

Box 906 illustrates transporting thermal energy from one region of thetitanium substrate to another region of the titanium substrate bydriving the fluid within the plurality of titanium pillars withcapillary motion.

ADVANTAGES OF THE PRESENT INVENTION

Titanium provides several material properties that are desirable interms of fracture toughness, strength-to-weight ratio, corrosionresistance, and bio-compatibility. For example, titanium has a fracturetoughness almost ten times that of diamond, and over 50 times that ofsilicon. Further, titanium is easily machined on both a macro and microscale, and can be welded easily to form hermetically sealed structures.

Further, titanium can be oxidized to form NST, which can increase thehydrophilicity of the wicking structure, and can be electroplated withvarious materials to increase the thermal conductivity of the wickingstructure, which provides for extreme design flexibility in the designof the wicking structure properties and characteristics.

Because titanium has a high fracture toughness, low coefficient ofthermal expansion compared to other metals, and a low modulus ofelasticity, the structure 100 can be manufactured to comply thermallyand physically with a large number of devices, including semiconductordevices, and the control of the dimensions and materials used withinwicking structure 105 allows for engineering of the performance of thewicking structure 105 for a wide range of applications.

However, the low thermal conductivity of titanium as compared to othermaterials, e.g., copper, gold, silicon carbide, etc., has previouslymade titanium a poor choice for use as a heat transfer mechanism or as athermal ground plane. Peer review of the present invention, whenproposed as a research project, resulted in a rejection of the use oftitanium as unpractical in thermal ground plane applications. However,the present invention shows that despite the low thermal conductivity oftitanium, the use of pillars 104, various composite materials within thewicking structure 105, and the controllability and the option to varythe of the size, shape, spacing (or gap), and pitch of the pillars 104within the plurality of pillars allows titanium to overcome thepreviously thought of deficiencies.

CONCLUSION

The present invention describes titanium-based thermal ground planes. Athermal ground plane in accordance with the present invention comprisesa titanium substrate comprising a plurality of pillars, wherein theplurality of Ti pillars can be optionally oxidized to formnanostructured titania coated pillars, and a vapor cavity, incommunication with the plurality of titanium pillars, for transportingthermal energy from one region of the thermal ground plane to anotherregion of the thermal ground plane. Such a thermal ground plane furtheroptionally comprises the titanium substrate that can optionally bethinned in at least part of an area of the substrate opposite theplurality of pillars, the vapor cavity being enclosed using a secondsubstrate (which can optionally be constructed from titanium), theplurality of titanium pillars being formed using optionally titaniuminductively-coupled-plasma etching, at least one characteristic of theplurality of pillars can be controlled and optionally varied within theplurality of pillars to adjust a thermal transport of the thermal groundplane, and the at least one characteristic being selected from a groupcomprising a height, a diameter, a spacing (or a gap), an amount ofoxidation, a pitch of the plurality of pillars, and a composition ofmaterial(s) which may include but is not limited to Ti, TiO₂, Au, or Cuapplied to the pillar surface to control surface physical propertiesincluding wettability.

A method of forming a thermal ground plane in accordance with thepresent invention comprises etching a titanium substrate to form aplurality of titanium pillars, optionally oxidizing the titaniumsubstrate to form nanostructured titania on the plurality of titaniumpillars, and forming a vapor cavity in contact with the plurality oftitanium pillars.

Such a method further optionally comprises the titanium substrate thatcan optionally be thinned in at least part of an area of the substrateopposite the plurality of pillars, the vapor cavity being enclosed usinga second substrate (which can optionally be constructed from titanium),the plurality of titanium pillars being formed using optionally titaniuminductively-coupled-plasma etching, at least one characteristic of theplurality of pillars being controlled and optionally varied to adjust athermal transport of the thermal ground plane, and the at least onecharacteristic being selected from a group comprising a height, adiameter, a spacing (or gap), an amount of oxidation, a pitch of theplurality of pillars, and a composition of material(s) which may includebut is not limited to Ti, TiO₂, Au, or Cu applied to the pillar surfaceto control surface physical properties including wettability.

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A thermal ground plane, comprising: a wicking structure formed in atitanium substrate, wherein the titanium substrate has a thickness andthe thickness is 25 micrometers-500 micrometers; and a vapor cavity incommunication with the wicking structure; wherein: a fluid containedwithin the wicking structure and the vapor cavity transports thermalenergy between a hotter region of the wicking structure and a colderregion of the wicking structure, the hotter region is hotter than thecolder region, the fluid is driven by capillary forces within thewicking structure, and the fluid comprises a liquid phase and a vaporphase.
 2. The thermal ground plane of claim 1, wherein: the wickingstructure comprises microfabricated structures and gaps between themicrofabricated structures, and the gaps in the colder region aredifferent from the gaps in the hotter region.
 3. The thermal groundplane of claim 1, wherein the wicking structure comprisesmicrofabricated structures and spacings between the microfabricatedstructures, and the spacings are at least 1 micrometer.
 4. The thermalground plane of claim 3, wherein the microfabricated structures eachhave a height of 5-200 micrometers and the spacings are 1-500micrometers.
 5. The thermal ground plane of claim 3, wherein themicrofabricated structures comprise etched microfabricated structures.6. The thermal ground plane of claim 1, wherein the wicking structurecomprises microfabricated structures and gaps between themicrofabricated structures, and and the fluid is transported through thegaps.
 7. A structure comprising: a first titanium substrate having athickness, wherein the thickness is 25 micrometers-500 micrometers; anda wicking structure formed into the first titanium substrate.
 8. Thestructure of claim 16, wherein the first titanium substrate and thesecond titanium substrate are welded together to form ahermetically-sealed thermal ground plane.
 9. The structure of claim 16,wherein the first titanium substrate and the second titanium substrateare laser welded together to form a hermetically-sealed thermal groundplane.
 10. A method of fabricating a thermal ground plane, comprising:forming a wicking structure in a titanium substrate, wherein thetitanium substrate has a thickness and the thickness is 25micrometers-500 micrometers; and coupling a vapor cavity with thewicking structure; such that a fluid contained within the wickingstructure and the vapor cavity transports thermal energy between ahotter region of the thermal ground plane and a colder region of thethermal ground plane, wherein the hotter region is hotter than thecolder region, the fluid is driven by capillary forces within thewicking structure, and the fluid comprises a liquid phase and a vaporphase.
 11. The method of claim 10, wherein: the wicking structurecomprises a plurality of microfabricated structures comprising titanium,and an etched surface of the microfabricated structures includes aroughness of 1-1000 nanometers.
 12. The method of claim 10, wherein: thewicking structure comprises microfabricated structures and spacingsbetween the microfabricated structures, and the spacings are at least 1micrometer.
 13. The method of claim 12, wherein the microfabricatedstructures each have a height of 5-200 micrometers and the spacings are1-500 micrometers.
 14. The method of claim 10, wherein: the wickingstructure comprises microfabricated structures and gaps between themicrofabricated structures, and the fluid is transported through thegaps.
 15. The method of claim 10, wherein the forming comprises etching.16. The structure of claim 7, further comprising a second titaniumsubstrate; a vapor cavity formed into the second titanium substrate,wherein the vapor cavity is in communication with the wicking structure;wherein: a fluid contained within the wicking structure and the vaporcavity transports thermal energy between a hotter region of the wickingstructure and a colder region of the wicking structure, the hotterregion is hotter than the colder region, the fluid is driven bycapillary forces within the wicking structure, and the fluid comprises aliquid phase and a vapor phase.
 17. The structure of claim 7, wherein:the wicking structure comprises microfabricated structures and spacingsbetween the microfabricated structures, and the spacings are at least 1micrometer.
 18. The structure of claim 17, wherein the microfabricatedstructures each have a height of 5-200 micrometers and the spacings are1-500 micrometers.
 19. The structure of claim 7, wherein: the wickingstructure comprises microfabricated structures and spacings between themicrofabricated structures, and the microfabricated structures compriseetched microfabricated structures.
 20. The manufacture of claim 19,wherein an etched surface of the etched microfabricated structuresincludes a roughness of 1-1000 nanometers.